1. Field of the Invention
The present invention relates to a focus detecting apparatus for detecting the focus of a camera and the like.
2. Related Background Art
In the past, a focus detecting apparatus as shown in FIG. 9 has already been known. In such conventional focus detecting apparatus, a zone (a)-(c) situated on a focus detecting plane 1 and having a center (b) positioned on an optical axis LX of a phototaking lens ("taking lens") with an extent of the order of +2-3 mm is used as a focus detecting area, and an image in this area is focused on image sensors 3a and 3b positioned on an IC substrate 3 through the medium of a pair of re-focusing lenses 2a and 2b, whereby a focus of the taking lens is determined by detecting the relative image displacement on the basis of image from the image sensors 3a, 3b.
In a focus detecting apparatus for a single lens reflex camera, it is necessary to prevent vignetting from occurring in an optical focus detecting system in the case of an interchangeable lens having an aperture F value of the order of F 5.6. In many cases, a position of an exit pupil of a taking lens having the aperture F value of F 5.6 exists anywhere in a range of 50 mm-200 mm, as shown in a hatched zone. In order to avoid vignetting in the lens having the aperture F value of F 5.6 wherein the position of the exit pupil exists in such range, it is necessary that the divergence .alpha. of a bundle of the detecting light is set to have a value of F 7 or thereabout.
Further, when the aperture or openings of the re-focusing lenses 2a, 2b are projected on the position corresponding to the exit pupil position of 100 mm or thereabout (a range shown by L.sub.1 in FIG. 9) by means of a field lens 6, it will be possible to detect the focus of the lens having the aperture value of F 5.6 wherein the position of the exit pupil exists within the range of 50 mm-200 mm, without the occurrence of vignetting, by the use of light beams (shown by broken lines) passing through the points (a), (c) corresponding to the positions of the image heights of 2-3 mm measured from the optical axis LX. With such arrangement, in the conventional focus detecting apparatus, it is possible to detect the focus of the lens without the occurrence of vignetting within the range corresponding to the image height of 3 mm or thereabout.
It is desired that the focus detecting be performed by the use of a focus detecting area positioned away from the optical axis of the lens by 3-4 mm or more. To this end, for example, as shown in FIG. 9, re-focusing lenses 4a, 4b and IC substrate 5 are arranged to obtain a focus detecting area included in a zone (d)-(e)-(f) having a central point (e) corresponding to the position of the image height of 7 mm. Now, this focus detecting optical system is so designed that the apertures of the re-focusing lens 4a, 4b provide a conjugate image on a position corresponding to the exit pupil position of substantially 100 mm with the aid of the field lens 6. In this case, as apparent from FIG. 9, it is possible to avoid vignetting for the taking lens having the aperture value of F 5.6 by the use of the focus detecting area (d)-(e)-(f), only when the position of the exit pupil of the taking lens exists in the range L.sub.1 having a value of 100 mm or thereabout. Accordingly, in this case, the kind of available interchangeable lenses is limited, and, thus, many problems arise in practice.